1. Field of the Invention
The invention relates to a digitizer and, in particular, to a digitizer with a variable sampling clock and communication system comprising the same.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a conventional digital TV receiver. The digital receiver 100 comprises a tuner 110, a demodulator 120, and a signal processor 130. The demodulator 120 comprises an analog to digital converter (ADC) 125. The tuner 110 receives a radio frequency (RF) signal and downconverts the same to an intermediate frequency (IF) signal. The ADC 125 in the demodulator 120 samples the IF signal and generates a digital signal. The digital signal is further demodulated in the demodulator 120 and then transmitted to the signal processor 130 for subsequent signal processing.
FIG. 2 is a schematic diagram of frequency distribution of channels before and after sampling of the ADC 125 in FIG. 1. The schematics above the frequency axis illustrate frequency distribution of channels in the IF signal before sampling of the ADC 125 and below the frequency axis illustrate frequency distribution of channels after sampling of the ADC 125. Shadow regions show location of desired (in-channel) signals of a channel N. Locations of unwanted adjacent channels are indicated by N+1, N+2, . . . and N−1, N−2, . . . . As an example, to comply with television standard of USA, the desired channel N in the IF signal typically centers at 44 MHz with a bandwidth of 6 MHz. According to the Nyquist criterion, the IF signal can be digitally sampled without alias effect only if the sampling frequency is larger than the Nyquist frequency, namely, twice its highest frequency component, or for this situation, larger than 94 MHz. It is, however, impractical to sample the IF signal at a frequency larger than the Nyquist frequency. Known practices call for sampling frequencies smaller than the Nyquist frequency. This sampling introduces alias effect, which results in overlapping unwanted signals into the desired signal spectrum after subsampling. The alias effect results in poor receiving condition because the overlapping unwanted signals interfere with and deteriorate the desired signal. As an example, after 25 MHz sampling of the ADC 125, various channels are folded into a sampling window ranging from −12.5 MHz to +12.5 MHz. As shown in FIG. 2, adjacent channels N+2 and N−2 have the same frequency as the desired channel N after sampling. If power of adjacent channels N+2 and N−2 is high, the desired channel N is affected thereby and the demodulator 120 may not work properly.